1. Field of the Invention
The present invention is within the field of electrographical printing devices in which a modulated stream of pigment particles is transported from a particle source through an apertured printhead structure and deposited in image configuration onto an image receiving medium. More specifically, the invention relates to an improvement of the properties of said pigment particles in order to obtain an improved printing result in a direct printing process.
2. Description of the Related Art
Of the various electrostatic printing techniques, the most familiar and widely utilized is that of xerography, wherein latent electrostatic images formed on a charge retentive surface, such as a roller, are developed by a toner material to render the images visible, the images being subsequently transferred to plain paper. This process is called an indirect printing process since the images are first formed on an intermediate photoreceptor and then transferred to a paper surface.
Another form of electrostatic printing is known as direct electrostatic printing (DEP). Many of the methods used in DEP, such as particle charging, particle transport, and particle fusing are similar to those used in xerography. However, DEP differs from xerography in that an electric field is generated by electrical signals to cause toner particles to be deposited directly onto plain paper to form visible images without the need for those signals to be intermediately converted to another form of energy. The novel feature of the DEP concept is the simultaneous field imaging and toner transport to produce visible images directly onto plain paper or any suitable image receiving medium.
U.S. Pat. No 5,036,341 granted to Larson discloses a DEP printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. The Larson patent discloses a method which positions a control electrode array between a back electrode and a rotating particle carrier. An image receiving substrate, such as paper, is then positioned between the back electrode and the control electrode array.
An electrostatic field on the back electrode attracts the toner particles from the surface of the toner carrier to create a particle stream toward the back electrode. The particle stream is modulated by voltage sources which apply an electric potential to selected individual control electrodes to create electrostatic fields which either permit or restrict the transport of toner particles from the particle carrier through the control electrode array. In effect, these electrostatic fields "open" or "close" selected apertures in the control electrode array to the passage of toner particles by influencing the attractive force from the back electrode. The modulated stream of charged toner particles allowed to pass through the opened apertures impinges upon a print-receiving medium interposed in the particle stream to provide line-by-line scan printing to form a visible image.
The control electrode array of the above-mentioned patent may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or a screen shaped, apertured printed circuit. Generally, the array is formed of a thin substrate of electrically insulating material provided with a plurality of apertures each of which is surrounded by an individually addressable control electrode, and a corresponding voltage source is connected thereto for attracting the charged toner particles from the particle carrier to the image receiving substrate by applying voltage signals in accordance with the image information. For example, the control electrode array may be constructed of a flexible, non-rigid material and overlaid with a printed circuit such that apertures in the material are arranged in several rows and surrounded by electrodes.
Toner particles are held on the surface of the particle carrier by an adhesion force which is substantially related to the particle charge and to the distance between the particle and the surface of the particle carrier. The electrostatic field applied on a control electrode to initiate toner transport through a selected aperture is chosen to be sufficient to overcome the adhesion force to cause the release of an appropriate amount of toner particles from the particle carrier. The electrostatic field is applied during the time period required for these released particles to reach sufficient momentum to pass through the selected aperture, whereafter the transported toner particles are exposed to the attraction force from the back electrode and intercepted by the image receiving substrate.
Properties, such as charge amount, charge distribution, particle diameter, etc., of the individual toner particles have been found to be of particularly great importance to print performance in a direct printing method. Accordingly, the size and size-distribution of the toner particles affects the printing result, since large toner particles have a tendency to cause clogging of the apertures in the control electrode array. In addition, the toner particles allowed to pass through selected opened apertures are accelerated toward the transfer belt under the influence of a uniform attraction field from the back electrode. In order to control the distribution of transported particles onto a printing substrate, the particles may be deflected by the application of a deflection pulse, resulting in an increase in the addressable area on the printing substrate. However, small particles having a low surface charge exhibit poor deflection properties.
Moreover, variations in charge amount and charge distribution affect the print uniformity and print quality of a direct printing method. Particularly, a non-uniform charge distribution on the surface of the particles may cause the formation of highly charged areas, or "hot spots" on the particle surfaces. Such hot spots are highly undesirable, since they may cause the toner particles to arrange themselves into chains or particle networks, resulting in an increased tendency to form clusters. Furthermore, the charge distribution has been found to affect the release of toner particles from the particle carrier. A non-uniform particle distribution also increases clogging of the apertures in the control electrode array. Accordingly, a non-uniform charge distribution and the formation of hot spots on the surface of the toner particles result in impaired release properties and clogging. Another negative effect of a non-uniform charge-distribution is increased dispersion of toner particles in the printing process.
To meet the requirements of higher resolution printing, such as for example 600 dpi printing, wherein the dot size is to be in the order of 60 microns, it is essential to provide DEP methods with improved dot size control, while ensuring minimal toner particle dispersion. Therefore, a more uniform characteristic and a smaller average diameter is required for the toner to efficiently improve the print quality, resolution and uniformity of DEP methods.